A radiation detector is a device that produces an output signal which is a function of the amount of radiation that is incident upon an active region of the detector. Infra-red detectors are those detectors which are sensitive to radiation in the infra-red region of the electromagnetic spectrum. There are two types of infra-red detectors, thermal detectors including bolometers and photon detectors.
The photon detectors function based upon the number of photons that are incident upon and interact with electrons in a transducer region of the detector. The photon detectors, since they function based on direct interactions between electrons and photons, are highly sensitive and have a high response speed compared to the bolometers. However, they have a shortcoming in that the photon detectors operate well only at low temperatures, necessitating a need to an incorporate therein an additional cooling system.
The bolometers function, on the other hand, based upon a change in the temperature of the transducer region of the detector due to absorption of the radiation. The bolometers provide an output signal, i.e., a change in the resistance of materials (called bolometer elements), that is proportional to the temperature of the transducer region. The bolometer elements have been made from both metals and semiconductors. In metals, the resistance change is essentially due to variations in the carrier mobility, which typically decreases with temperature. Greater sensitivity can be obtained in high-resistivity semiconductor bolometer elements in which the free-carrier density is an exponential function of temperature.
In FIGS. 1 and 2, there are shown a perspective view and a cross sectional view illustrating a three-level bolometer 100, disclosed in U.S. application Ser. No. 09/102,364 entitled "BOLOMETER HAVING AN INCREASED FILL FACTOR". The bolometer 100 comprises an active matrix level 10, a support level 20, a pair of posts 40 and an absorption level 30.
The active matrix level 10 has a substrate 12 including an integrated circuit (not shown), a pair of connecting terminals 14 and a protective layer 16. Each of the connecting terminals 14 made of a metal is located on top of the substrate 12. The protective layer 16 made of, e.g., silicon nitride (SiN.sub.x) , covers the substrate 12. The pair of connecting terminals 14 are electrically connected to the integrated circuit.
The support level 20 includes a pair of bridges 22 made of silicon nitride (SiN.sub.x) , each of the bridges 22 having a conduction line 24 formed on top thereof. Each of the bridges 22 is provided with an anchor portion 22a, a leg portion 22b and an elevated portion 22c, the anchor portion 22a including a via hole 26 through which one end of the conduction line 24 is electrically connected to the connecting terminal 14, the leg portion 22b supporting the elevated portion 22c .
The absorption level 30 is provided with a bolometer element 32 surrounded by an absorber 31 and an IR absorber coating 33 formed on top of the absorber 31. The absorber 31 is fabricated by depositing silicon nitride before and after the formation of the bolometer element 32 to surround the bolometer element 32. Titanium (Ti) is chosen as the material for bolometer element 32 because of the ease with which it can be formed. Serpentine shape gives the bolometer element 32 to high resistivity.
Each of the posts 40 is placed between the absorption level 30 and the support level 20. Each of the posts 40 includes an electrical conduit 42 made of a metal, e.g., titanium (Ti), and surrounded by an insulating material 44 made of, e.g., silicon nitride (SiN.sub.x) Top end of the electrical conduit 42 is electrically connected to one end of the serpentine bolometer element 32 and bottom end of the electrical conduit 42 is electrically connected to the conduction line 24 on the bridge 22, in such a way that each ends of the serpentine bolometer element 32 in the absorption level 30 is electrically connected to the integrated circuit of the active matrix level 10 through the electrical conduits 42, the conduction lines 24 and the connecting terminals 14.
When exposed to infra-red radiation, the resistivity of the serpentine bolometer element 32 increases, causing a current and a voltage to vary, accordingly. The varied current or voltage is amplified by the integrated circuit, in such a way that the amplified current or voltage is read out by a detective circuit (not shown).
In the above-described infrared bolometer, in order to decrease the thermal exchange between the active matrix level and the absorption level, the support level is as long as possible, and this is achieved by cantilevering the support level on the active matrix level. This solution, however, has a drawback in that the elevated portion of the bridge gets warped easily and bent upward to relieve the elastic stress accumulated in the bridge during the formation thereof, which will, in turn, bend the absorber, resulting in decreasing the absorbing efficiency of the bolometer.